A continuous rotation servo is a compact motor module that looks like a hobby servo but spins like a small geared motor. It is useful in robotics because it combines a DC motor, gear train, and control electronics in one easy-to-mount package. Instead of moving to a fixed angle, it turns forward or backward at a speed set by a control pulse.
This makes it common in small wheeled robots, conveyor models, and rotating mechanisms.
The control signal is usually a pulse-width modulation signal with pulses about every 20 ms. A pulse near 1.5 ms commands stop, shorter pulses command rotation one way, and longer pulses command rotation the other way. The servo electronics compare the signal to a neutral point and drive the internal DC motor through gears to produce useful torque at the output shaft. Because it is designed for continuous spin, it does not know its exact angular position unless extra sensors are added.
Key Facts
- A standard servo controls angle, while a continuous rotation servo controls speed and direction.
- Typical control period: T = 20 ms, or frequency f = 1/T = 50 Hz.
- Neutral command is often about 1.5 ms pulse width, which should make the servo stop.
- Pulse width less than 1.5 ms usually spins one direction, and pulse width greater than 1.5 ms usually spins the opposite direction.
- Duty cycle = pulse width / period, so a 1.5 ms pulse in a 20 ms period has duty cycle = 0.075 = 7.5%.
- Wheel speed relates to robot speed by v = 2πrN, where r is wheel radius and N is wheel rotations per second.
Vocabulary
- Continuous rotation servo
- A modified servo motor that rotates continuously, with its speed and direction controlled by pulse width.
- Pulse-width modulation
- A control method that changes the width of repeated electrical pulses to send a command to a device.
- Neutral pulse
- The pulse width that tells a continuous rotation servo to stop spinning, usually near 1.5 ms.
- Gear train
- A set of gears that reduces motor speed while increasing torque at the output shaft.
- Torque
- A twisting force that causes rotation, measured in units such as newton-meters or kilogram-centimeters.
Common Mistakes to Avoid
- Treating a continuous rotation servo like a position servo is wrong because pulse width controls speed and direction, not a target angle.
- Assuming 1.5 ms always stops the servo exactly is wrong because the neutral point can vary and may need adjustment or calibration.
- Powering the servo from a microcontroller signal pin is wrong because the signal pin cannot supply the motor current needed by the servo.
- Forgetting a common ground is wrong because the servo and controller need the same voltage reference for the signal pulses to be interpreted correctly.
Practice Questions
- 1 A controller sends pulses every 20 ms. What is the frequency of the PWM signal in hertz?
- 2 A continuous rotation servo receives a 1.2 ms pulse every 20 ms. What is the duty cycle as a percent, and is the command likely stop, forward, or reverse if 1.5 ms is neutral?
- 3 A two-wheel robot uses continuous rotation servos. Explain why matching the same pulse width on both servos may not make the robot drive perfectly straight.